A scanning electron microscope (SEM) is a type of electron microscope that images a sample by scanning it with a focused beam of electrons in a predetermined pattern. The electrons interact with the atoms that make up the sample producing signals that provide information about the sample's surface topography, composition, and other properties.
The types of signals produced by an SEM include secondary electrons, back-scattered electrons (BSE), characteristic X-rays, light (cathodoluminescence), specimen current and transmitted electrons. The signals result from interactions of the electron beam with atoms at or near the surface of the sample. In one common detection mode, secondary electron imaging (SEI), the SEM can produce very high-resolution images of a sample surface, revealing details less than 1 nm in size. Due to the very narrow electron beam, SEM micrographs have a large depth of field yielding a characteristic three-dimensional appearance useful for understanding the surface structure of a sample.
Scanning electron microscopes are used in mineral analysis systems, such as the Qemscan® and MLA® available from FEI Company, Hillsboro, Oreg., have been used for many years to analyze mineral samples. To determine the type and relative quantity of minerals present in a mine, a sample in the form of small granules, is fixed in epoxy in a mold and the mold is placed in a vacuum chamber. An electron beam is directed toward a sample and, in a process called x-ray spectroscopy, the energies of x-rays coming from the sample in response to the electron beam are measured and plotted in a histogram to form a spectrum. The measured spectrum can be compared to the known spectra of various elements to determine which elements and minerals are present. One type of x-ray spectroscopy is “energy dispersive x-ray spectroscopy” or “EDS.” Another type is “wavelength dispersive spectroscopy,” or “WDS.”
To stimulate the emission of characteristic X-rays from a specimen, a high-energy beam of charged particles such as electrons or protons, or a beam of X-rays, is focused into the sample being studied. At rest, an atom within the sample contains ground state (or unexcited) electrons in discrete energy levels or electron shells bound to the nucleus. The incident beam may excite an electron in an inner shell, ejecting it from the shell while creating an electron hole where the electron was. An electron from an outer, higher-energy shell then fills the hole, and the difference in energy between the higher-energy shell and the lower energy shell may be released in the form of an X-ray. The unique atomic structure of each element allows x-rays that are characteristic of an element's atomic structure to be uniquely identified from one another. The number and energy of the X-rays emitted from a specimen can be measured by an x-ray spectrometer, such as an EDS or a wavelength dispersive spectrometer, to determine the elemental composition of the specimen to be measured.
Back-scattered electrons (BSE) are electrons from the primary electron beam that are reflected from the sample by elastic or inelastic scattering. BSE are often used in analytical SEM along with the spectra made from the characteristic X-rays, because the intensity of the BSE signal is strongly related to the atomic number (Z) of the specimen. BSE images can provide information about the distribution of different elements in the sample.
BSE signals are typically acquired at a rate of microseconds per pixel. EDS systems have a longer acquisition time, typically requiring on the order of several seconds per pixel to produce a spectrum having sufficient resolution to uniquely identify a mineral. The longer time required to collect an x-ray spectrum to uniquely identify a mineral substantially limits the number of pixels that can be measured. EDS systems are also typically insensitive to light atoms. Because of the advantages of both EDS detectors and BSE detectors, it is sometimes useful to use both BSE and x-ray spectra to accurately identify minerals, which requires more time and becomes a difficult problem to solve with a commercially viable approach.
The Qemscan® and MLA® systems comprise an SEM, one or more EDS detectors, and software for controlling data acquisition. These systems identify and quantify elements represented within an acquired spectrum, and then match this elemental data against a list of mineral definitions with fixed elemental ranges.
Some mineral classification systems compare the acquired spectrum of an unknown mineral to a library of known mineral spectrums, and then identify the sample based on which known spectrum that is most similar to the sample's spectrum. There are various ways to compare two spectrums directly. For example, one can take the sum of the differences between the two spectrums at different energy channels as a “distance.” The MLA, uses a chi-squared statistical test to compare the value at each energy channel of the measured spectrum to the value at the corresponding channel of the known mineral spectrum. The MLA assigns the unknown spectrum to the element having the “best match” to the spectrum, as long as a minimum similarity is met.
QEMSCAN, on the other hand, uses a system of rules or criteria that, if met, classify the spectrum. This is typically applied in a “first match” manner, that is, the spectrum is compared sequentially to the criteria for each possible mineral, and when the spectra meets a criteria, the system assigns that element to the spectrum.
Existing systems require expert users to spend time i) locating and collecting examples of minerals to use as standards to compare against the unknown spectrum (MLA) or ii) formulating a list of rules that are applied sequentially to identify a mineral (QEMSCAN). These requirements have made it difficult to make an automated mineral identification system or even a system that can be used by relatively unskilled operators.